The present invention relates to an apparatus for tracking an optical disc by means of sampling marks regularly distributed around the track and, more particularly, to an apparatus for tracking an optical disc capable of pulling into a tracking control operation steadily and quickly.
In one known method for tracking an optical disc, reproduced signals are obtained from pits previously formed distributed around the track at regular intervals and a tracking signal is obtained based on the reproduced signals from the pits. That is, as described in U.S. Pat. No. 4,562,564, pairs of pits are previously formed along an imaginary center line of the track in a wobbled manner, and the tracking error signal is obtained based on the fact that the signal amplitude values obtained from the wobbled pits change according to relative distance between the light beam and the wobbled pits. A similar tracking method is also discussed in a paper published in SPIE, Proceeding, Vol. 695, Optical Data Storage II (1986), pp. 112-115. This method uses sampling marks consisting of three pits, as shown in FIG. 1. Referring to the figure, reference numerals 50 and 51 indicate a pair of pits for detecting a tracking error signal, which are offset from the imaginary center 53 of the track to opposite sides by a distance of 1/4 of a track pitch. Reference numeral 52 indicates a pit located on the imaginary center of the track for reproducing a clock signal. FIG. 1 shows three tracks a, b, c, on which pits 50, 51, 52 are distributed respectively. On each track, a distance between pits 50 and 51, and a distance between pits 51 and 52 are a constant as (T). The pits 50, 51 and 52 are arranged on the track repeatedly with a same order.
The tracking error signal which is generated when the track moves in a direction of an X-axis with a high speed in accordance with a rotation of the disc, and when a light beam moves across the tracks on the optical disc in a direction of a Y-axis perpendicular to the track is made up of the difference between amplitude values of the reproduced signals from the pits 50 and 51. That is, such a sinusoidal tracking error signal as shown in FIG. 2, wherein points a, b, c represent that an optical pick-up is located on the center of the track and the points d, e represent that an optical pick-up is located in the midpoints between tracks. At the points a, b, c, the amplitude values of the signals from the pits 50 and 51 are equal to each other, whereby the tracking error signal becomes zero.
At the points d, e, the amplitude values of the signals from the pits 50 and 51 of neighboring tracks become equal to each other, and therefore, an apparently similar tracking error signal to that generated in the center of a track is obtained. But, the polarity of the signal is opposite to that. Hence, it results in a positive feedback for the servo system, and therefore, when the tracking servo system is closed while the light spot is located between tracks, it may occur that the servo system oscillates and becomes unable to achieve alignment of the light spot of the track. Thus, a long time may be required for acquiring a steady tracking condition at the time of access.
In the above described prior art, particular consideration has not been given to the problem of such a positive feedback in the tracking servo system. As a result, there was a drawback that a long time was required before acquiring the steady tracking condition.
List of relative prior art patents and a prior application:
4,562,564, 4,561,082, 4,553,228, 4,432,083, 4,489,406, 4,435,797, 4,402,061, and 4,443,870, U.S. patent application filed on Apr. 21, 1988 corresponding to Japanese Patent Application No. 99707/87